The subject matter of WO 00/11684, not published before the priority date of the present application, discusses a method for producing a sheathing made of an electrical insulating material of plastic on all sides around at least one superconductor with high-Tc superconductor material. According to this proposed method, it is intended to provide a continuous sheathing process at a process temperature having virtually no detrimental effect on the superconducting properties of the conductor by                the conductor emerging from a guide channel extending in a direction of advancement,        extruding a melt tube of molten thermoplastic insulating material in the direction of advancement from a die, the outlet opening of which surrounds the conductor at a distance on all sides,        the melt tube being stretched and drawn onto the surface of the conductor as the conductor is advanced and        the melt tube applied in this way to the surface of the conductor being made to set by cooling.        
This proposed method is intended to be used in particular for sheathing a superconductor in strip form with an aspect ratio of at least 3, preferably at least 10.
To allow them to be used in electrical devices, such as windings of machines, transformers, magnets or cables, industrial superconductors must generally be provided with an electrical insulation. Such a requirement also exists in particular in the case of conductors with oxidic high-Tc superconductor material (HTS material). In this case it is intended that such HTS conductors, which may be of a wire form (with circular cross section) and in particular of a strip form (with rectangular cross section), can be provided continuously with an insulating sheathing in a method which is simple to carry out. The method is intended in this case to be suitable both for single-conductor insulation and for the insulation of an HTS conductor construction in the form of a multiple conductor, which is composed of individual superconducting conductors, or a composite conductor with superconducting and normally conducting parts.
There have not in the past been any known methods realized on an industrial scale by which a superconductor or conductor construction with HTS material can be provided on all sides with an insulating sheathing while it continuously runs through. One of the reasons for this is that the currently pursued HTS conductor concepts provide a strip form with an unfavorably high aspect ratio (=ratio of conductor width to conductor thickness) with regard to insulating methods practiced in superconducting technology. This is so because it is only possible with difficulty for such conductors to be uniformly coated with a small thickness of an insulating material by the known methods. In the case of an HTS conductor disclosed by EP 0 292 126 B1, the sheathing is therefore made relatively thick.
Classic coating methods have previously been unsuitable for HTS conductors because they can lead to a current degradation of the conductor, which is the consequence of the high process temperatures, required for these methods, and of supercritical bending stresses, which occur when the conductor is passed periodically through immersion baths with multiple deflection by corresponding deflecting rollers.
To make it possible for known HTS strip conductors in strip form to be used in the construction of magnetic windings, for example, in the past separate insulating films, for example of a special aromatic polyamide known by the trade name “Kapton”, and having a thickness of, for example, 50 μm, have been wound together with the strip conductor. Consequently, apart from an unwinding device for the conductor, a corresponding device for the insulating film has to be additionally provided for the production of windings, in order to produce an insulation between the individual layers or turns of a winding. In this case, the difficulty may arise that the conductor is not completely sheathed by the insulating film. Furthermore, there is in each case only one separating layer between the individual conductor layers, with the lateral edges of the conductor remaining uninsulated. To ensure reliable insulation in these regions also, either casting of the wound assembly with casting resin or the use of insulating films wide enough for a short-circuit between the conductors to be prevented by a lateral overhang of the film beyond the respective conductor edges is necessary. However, the adjusting effort to make it possible for the conductor and insulating film to be wound in parallel is relatively high.
In addition, it is known from the technique of insulating superconductors with what is known as classic superconductor material, which require an LHe cooling technique, to wrap a superconductor in strip form, for example, with a corresponding film of plastic (cf. DE 23 45 779 A or DE 38 23 938 C2). These methods can also only be carried out with relatively great effort. Furthermore, the films used must been of a sufficient thickness to rule out mechanical damage during the wrapping process.
Furthermore, it is also to be regarded as extremely difficult to spin insulating strip or insulating filaments around the very small cross sections of current HTS strip conductors with their typically large aspect ratio.
In the method according to WO 00/11684, not published before the priority date of the present application, for which the method features stated at the beginning are proposed, the application of a sheathing of thermoplastic insulating material takes place by using a thin-film extrusion technique based on what is known as a tube-stretching method. In this case, a melt tube is extruded from a die, which is larger in its dimensions than the conductor to be sheathed, which runs through a central guide channel in the center of the die. This produces a tube around the conductor, which is stretched, i.e. elongated, by the advancement of the conductor, until the final, desired thickness of the sheathing wall (insulating layer) is reached. This tube is drawn onto the surface of the conductor. Depending on the insulating material used, what is known as the degree of stretching, i.e. the stretching of the material, is in this case generally between 5 and 15. The stretching may advantageously take place with a vacuum simultaneously acting in the interior of the tube. Together with advantageous preheating of the conductor before entry into the guide channel and/or during the drawing of the conductor through the latter, in this way a particularly good and bubble-free bonding fit of the sheathing on the superconductor can be produced. The slow cooling then taking place, for example in air, brings about a solidification and stress-free setting of the melt of the insulating material on the conductor.
With this method, relatively thin (of a minimum thickness of approximately 40 μm and/or a maximum thickness of 100 μm) and defect-free sheathing layers can consequently be realized on superconductors of in fact any cross-sectional form, in particular however of strip form.
Known in principle are coating installations via which insulating sheathings of a thermoplastic material are to be applied to wires (cf. DE 26 38 763 A) through stripping dies, by pressure sheathing or by the tube-stretching method (DE 24 09 655 A, 20 22 802 A, DE 21 10 934 A). The wires may in this case consist in particular of steel (cf. U.S. Pat. No. 3,893,642), A1 (cf. DE 24 09 655 A) or Cu (cf. U.S. Pat. No. 4,489,130 or the cited DE 21 10 934 A) and generally have circular cross-sectional surface areas.
The coating method to be performed with such installations is also referred to as extrusion coating.
The proposed method is based on the realization that the aforementioned methods, known per se, are suitable for the coating of oxidic HTS conductors, allowing the conductor-specific difficulties mentioned at the beginning to be avoided. This is of significance, in particular, in the case of a strip form of the superconductor. In this context, a strip form is to be understood as meaning any desired rectangular form with angular or rounded edges. Preferably, however, the rectangular form may have a relatively large aspect ratio, generally above 10, as is the case in particular with known thin HTS strip conductors. By coating on the basis of the proposed tube-stretching method, pore-free insulating layers which adhere well on the surfaces typical of HTS conductors can be realized.
Applying this method to oxidic HTS conductors with their typical thermal and mechanical sensitivity opens up an extended area of applications for these types of conductors on account of the easier usability of already preinsulated conductors. Furthermore, considerable cost savings can be expected in comparison with the methods previously used in superconducting technology. Apart from the savings resulting from an efficient, rapid extrusion technique, there is considerable potential for rationalization in the usable insulating materials, which are significantly less expensive in comparison with known insulating films.
With the proposed method, continuous coating of an HTS conductor is possible, since the insulating material can be transported from a storage container which can be replenished at any time. Furthermore, with the method, the thickness of the insulating sheathing can be set variably in a wide range and with sufficient accuracy.
Since, for example, each individual conductor can be completely insulated, there is double insulation reliability in the case of strip conductor windings, because the conductors are separated by a twofold insulating layer. Furthermore, the use of different thermoplastic materials allows the combination of mechanical and thermal properties of the sheathing to be adapted to the respective application. In addition, the proposed method is significantly faster than a classic spinning or coating method previously used for metallic superconductors.
Furthermore, in the proposed method, the lateral conductor edges are also insulated, reducing the risk of short-circuits in this region. The insulation is also suitable in particular for thin strip conductors with an unfavorable aspect ratio. In this case there is no longer the risk feared when using coating methods of “edge recession”, i.e. undesired extreme thinning of the layer in the region of edges with small edge radii, as exist precisely in the case of thin conductor strips.
Furthermore, in the proposed method, the HTS conductor need not be mechanically loaded too much. This is because the mechanical loading is restricted to the small pulling forces produced by conductor unwinders or winders. The deflection of the conductor during the coating process can thus be advantageously avoided.
In the proposed method, known thermoplastic materials with a relatively low processing or melting temperature of below 200° C. are to be used and only relatively brief heating of the conductors is provided, to avoid, at least to a great extent, degradation of the superconducting properties (with respect to the critical temperature Tc and in particular with respect to the critical current density Jc, to be measured in A/m2). Proposed as thermoplastic materials suitable for this are polyethylenes, polystyrene-ethylene-butylene elastomers, polyurethane elastomers, ethylene/vinyl acetate copolymers or acrylic acid/acrylate copolymers.
With the thermoplastics listed above, insulating layer thicknesses of minimally about 40 to 50 μm can be realized. However, to achieve an effective current density that is as high as possible in a high-Tc superconductor and/or a device constructed with such conductors, such as for example a superconducting winding, the insulating layer should be smaller than this. In this case it should be possible to ensure good bonding of the insulating material on the conductor and good coupling of the corresponding insulating layer to impregnating and casting resins. It is found, however, that, with the proposed insulating materials, the production of what are known as Roebel bars (cf. for example DE-C 277012 or “Siemens Review”, Vol. 55, No. 4, 1988, pages 32 to 36, or “IEEE Transactions on Applied Superconductivity”, Vol. 9, No. 2, June 1999, pages 111 to 121), for example, is problematical, since these insulating materials are relatively soft at room temperature and have a high friction coefficient.